Membranes are widely used in a variety of applications to separate and purify liquid and gas streams. The most widely used material for the separation layer of membranes are polymers. Many polymeric materials have desirable membrane properties. However, many of these polymer materials can only be used by forming the polymer material on a support material. Exemplary support materials include polymers, plastics, metals, ceramics, and organic material. The support material may be porous or non-porous. The support material is generally required because the separation layer is thin and delicate.
Various methods have been used to form thin separation layers on support material. Traditional approaches involve solution-deposition, plasma polymerization, interfacial polymerization, and doctor blade approaches on flat sheets. These techniques may have various drawbacks, such as limited choice of applicable materials, high expense, and lack of coating durability.
Another method that has been used to form thin polymer separation layers on the surface of support materials is surface initiated radical polymerization. However, various issues arise with such techniques. For example, a free radical polymerization process initiated from a surface is generally not controllable. As a result, the polymer layer formed by this method may be too thick to function and/or may lack uniform thickness.
Controlled radical polymerizations initiated from a surface have meet with greater success. The slow and controlled nature of these reactions allows for uniform coatings to be formed. Unfortunately, previously available techniques of controlled radical polymerization involve the use of harsh conditions and/or environments that were not economically viable to scale.
Atom-transfer radical-polymerization, such as disclosed in U.S. Pat. Nos. 5,763,548 and 5,789,487, allows for controlled radical polymerization reactions to be performed using inexpensive reagents in mild conditions. Varieties of ATRP may be performed at room temperature and pressure with aqueous solvents.
The process of carrying out surface initiated atom-transfer radical-polymerization generally requires one or more preparation steps in which the surface on which a layer of material is formed is functionalized. Functionalization steps can include hydroxylating the surface and halogenating the surface. Various drawbacks associated with these functionalization steps have contributed to surface initiated atom transfer radical-polymerization not being usable on a commercial scale.
In the traditional hydroxylation step, gases such as ozone have been used to add —OH groups to the surface of a material. The use of ozone complicates the overall process because it can rapidly degrade the mechanical properties of the polymers upon exposure.
In the traditional halogenation step, acyl halides such as bromoisobutyrl bromide has been used. The use of such acyl halides poses difficulties due to the air and water sensitivity of these reagents (i.e., they environment must be kept air and water free). As a result, scale up of the halogenation process is difficult or impossible.
The problems identified above with functionalization have also posed problems with respect to scale up. To date, no methods of surface-initiated atom-transfer radical polymerization has been developed which can be used to add polymer coatings to surfaces at high volumes and with relatively low costs.